Input pull downs?

kenmac

It's normal with PIC's etc to install pulldown resistors on input pins (switching +ve).
Does this practice also apply to the propeller inputs, or is it catered for internally?

kenmac

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Perth, Western Australia
Time Zone = GMT + 8

Mike Green

Any unused CMOS input should be tied to Vss or Vdd via a pulldown/pullup resistor. If not, the input is high impedance and will tend to "wander around" generally in the middle of the input range where the input transistors are in their "linear" area. This results in relatively high currents in the input stages and wasting of power due to this. If you tie the input to Vss or Vdd, one of the two input transistors is "hard off" and there's minimum power use by the input stage.

Some microprocessors have built-in pullup or pulldown resistors. Some have configurable pullup resistors (like the SX as found in the BS2px pin configuration statements).

The Propeller does not have built-in pullups. Like any microprocessor with input/output pins, the unused pins can be configured as outputs during program initialization so they would be "floating" for only a short period of time. The unchanging outputs, as long as they're unconnected, don't draw significant current.

OzStamp

Hi KenMac
You need to use external pull ups ( if switching -) or external Pull downs
if switching +··
Pulling up inputs is more common.
So it is not catered for internally on the Propeller

The SX chips also from Parallax have a PLP register that you can
set ( you can only pull up ) on these chips.When pins are set to input ...

Rgds
Ronald Nollet

=========================================================
It's normal with PIC's etc to install pulldown resistors on input pins (switching +ve).
Does this practice also apply to the propeller inputs, or is it catered for internally?

kenmac
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Harrison.

If you are talking about pulldowns/pullups for switch inputs or signals where one of the states is hi-z (floating) then yes, it is required. The reason is that you don't want any spurious readings when the pin is in the floating state.

The Propeller does not have any internal pullups/pulldowns except on the RESn pin when BOEn is enabled. Most PICs have software configurable pullups usually on portB. I wouldn't rely on internal pullups anyways because they are usually weak.

Harrison

kenmac

Thanks folks - I just needed confirmation that they are needed.
I'll continue to use them.

kenmac

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Perth, Western Australia
Time Zone = GMT + 8

deSilva

kenmac said...
I just needed confirmation that they are needed.

No, they are not "needed"! Mike gave an excellent depiction of the situation three postings above, worth to be read twice...